Polishing a work, e.g., wafer, is performed by pressing a surface of the work to be polished onto a surface of a polishing pad adhered on a polishing plate and rotating the polishing plate with supplying a polishing liquid to the polishing pad.
However, clogging the polishing pad gradually occurs by performing the polishing many times, and a polishing rate will be lowered. Thus, the surface of the polishing pad is dressed, by a dressing grindstone, every time after performing the polishing prescribed times (see Patent Document 1).
Patent Document 1 discloses a method of flattening a semiconductor device, in which a dressing condition is controlled. The method is performed in an apparatus comprising: a dressing rate measuring unit for detecting a dressing rate of a polishing pad, which is varied with a progress of the polishing; and a surface property measuring unit for measuring a surface property of the polishing pad. The data automatically measured are used to maintain the dressing rate, which significantly influences on a scratch density, within prescribed management values.
In Patent Document 1, the method of measuring the surface property of the polishing pad is performed by an image processing manner or a reflectance manner. Namely, the image processing manner is performed by the steps of: lighting the surface of the polishing pad by a light emitting unit; producing an image of the lighted part by a CCD camera; and processing the image so as to calculate an area ratio of a flat part formed by the clogging. The reflectance manner is performed by the steps of: emitting a laser beam toward the surface of the polishing pad; receiving the reflected beam; and measuring the surface property of the polishing pad on the basis variation of light quantity of the received beam.